1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, it relates to a color image forming apparatus which can form a color image by combining a plurality of colors.
2. Description of the Related Art
For example, an electrophotographic color copying machine comprises developing units of color toners in a plurality of different colors, such as four colors of cyan (C), magenta (M), yellow (Y) and black (BL), for example. A multicolor copy is obtained by color-separating an original through a plurality of optical color filters, developing the same with color toners corresponding to the colors of the optical color filters and overlappingly repeating such development on the same paper with different toners. Such a color copying machine is provided with a color sensor for reading an original image by electrical means in order to improve black color reproducibility, for example. A portion of the original image determined to be in black is subjected to single color development of only black, with no development in other colors (refer to U.S. Pat. No. 4,655,579).
In an actual color copying machine, however, such single color development is required only in those portions having a relatively high spatial frequency such as a line image of characters or the like. In a portion of a picture etc. having a low spatial frequency, on the other hand, it is preferable to perform development by overlapping a plurality of colors, in order to improve reproducibility of color tones, etc.